Nitrogen has two kinds of stable isotopes, 14N and 15N. The natural abundance of 14N and 15N is 99.635 atom % and 0.365 atom % respectively. Moreover, “heavy nitrogen isotope” in the present invention means 15N.
There are three isotopic molecules of nitrogen, 14N2, 14N15N, and 15N2. The abundance ratios thereof are as follows.14N2: 99.635 atom %×99.635 atom %=99.271 mol %14N15N: 99.635 atom %×0.365 atom %×2=0.727 mol %15N2: 0.365 atom %×0.365 atom %=0.001 mol %
At present, 15N, which is a stable isotope of nitrogen, is used as a tracer in mainly natural science fields and medical fields. In addition, usage of 15N has also been examined in energy fields. However, the abundance ratio of 15N is extremely low. Therefore, a method for effectively concentrating 15N to a high concentration is desired. Examples of a method for concentrating 15N include a chemical exchange method which uses the phenomena by which the equilibrium distribution concentration of isotopes varies depending on the kinds of compound, a vapor adsorption method which uses the difference in adsorption behavior of ammonia to zeolite due to an isotope effect, and a low temperature distillation method which produces 15N and 18O by distilling nitrogen monoxide (NO) at low temperatures to concentrate 15N18O.
Examples of the method for concentrating isotopes by low temperature distillation include a carbon monoxide distillation method and a methane distillation method in which 13C is concentrated, in addition to the nitrogen monoxide distillation in which 15N and 18O are concentrated. Furthermore, a method, in which 18O is highly concentrated by combining oxygen distillation and isotope scrambling can also be used.